Currently, various power converters are developed toward directions of high efficiency, high power density, high reliability and low cost. Among many power converters, LLC series resonant converters have main switches operating under a Zero-Voltage Switching (ZVS) condition and rectifiers operating under a Zero-Current Switching (ZCS) condition, and can optimize conversion efficiency of high voltage section for an input voltage with a wide range, so they are widely used in high efficiency Direct Current to Direct Current (DC/DC) converters or Direct Current to Alternating Current (DC/AC) converters.
In order to realize a high voltage output and meanwhile to make a bus voltage withstood by the single input capacitor or the single switching element in a LLC series resonant converter not to be over high, a plurality of input capacitors may be connected in series between buses at an input side of the LLC series resonant converter.
FIG. 15 illustratively shows a schematic structure diagram of a LLC series resonant converter in conventional technologies. In the LLC series resonant converter, a first input terminal Vbus1 and a second input terminal Vbus2 are configured to input a DC bus voltage. Two input capacitors C11 and C12 are connected in series between the first input terminal Vbus1 and the second input terminal Vbus2. The input capacitor C11 is connected in parallel with a first bridge arm B11, and the input capacitor C12 is connected in parallel with a second bridge arm B12. The first bridge arm B11 includes switching elements Q11 and Q12 connected in series, and the second bridge arm B12 includes switching elements Q13 and Q14 connected in series. The first bridge arm B11 and the second bridge arm B12 are electrically coupled to output circuits O11 and O12, respectively. The output circuits O11 and O12 have the same structures and both employ a LLC series resonant circuit.
In the circuit shown in FIG. 15, each switching element in the single bridge arm only needs to withstand ½ bus voltage, and then a high voltage output can be realized. Thus, relatively cheap switching elements with low withstanding voltages, for example, 600V Metal Oxide Semiconductor Field effect Transistors (MOSFETs), may be chosen for realizing a high voltage output.
However, the structure shown in FIG. 15 has the following problems. In an ideal operating condition, the voltages across the two input capacitors C11 and C12 should be equal. However, because of limitations in the manufacturing process or other factors of practical switching elements, it is usually hard for the device parameters of switching elements to be completely consistent and they have more or less variations. Thus, turn-on time and turn-off time of different switching elements may vary, resulting in the difference of discharging time between the two input capacitors C11 and C12 and thereby causing the voltages across the input capacitors C11 and C12 to be unbalanced. The voltage imbalance will result in voltage difference between the switching elements, causing the switching elements to be damaged or even causing abnormal operation of the power converter.
In order to overcome the above problems, one of the methods is to employ hardware. For example, additional power converters may be employed to inject current into corresponding input capacitors or to draw current from corresponding capacitors so as to compensate the voltage imbalance. However, these methods will make the cost of the system remarkably increase.